eventfd: present id to userspace via fdinfo

Finding endpoints of an IPC channel is one of essential task to
understand how a user program works.  Procfs and netlink socket provide
enough hints to find endpoints for IPC channels like pipes, unix
sockets, and pseudo terminals.  However, there is no simple way to find
endpoints for an eventfd file from userland.  An inode number doesn't
hint.  Unlike pipe, all eventfd files share the same inode object.

To provide the way to find endpoints of an eventfd file, this patch adds
"eventfd-id" field to /proc/PID/fdinfo of eventfd as identifier.
Integers managed by an IDA are used as ids.

A tool like lsof can utilize the information to print endpoints.

Link: http://lkml.kernel.org/r/20190327181823.20222-1-yamato@redhat.comSigned-off-by: Masatake YAMATO <yamato@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/eventfd.c

@@ -21,6 +21,9 @@
 #include <linux/eventfd.h>
 #include <linux/proc_fs.h>
 #include <linux/seq_file.h>
+#include <linux/idr.h>
+
+DEFINE_IDA(eventfd_ida);
 
 struct eventfd_ctx {
 	struct kref kref;
@@ -35,6 +38,7 @@ struct eventfd_ctx {
 	 */
 	__u64 count;
 	unsigned int flags;
+	int id;
 };
 
 /**
@@ -69,6 +73,8 @@ EXPORT_SYMBOL_GPL(eventfd_signal);
 
 static void eventfd_free_ctx(struct eventfd_ctx *ctx)
 {
+	if (ctx->id >= 0)
+		ida_simple_remove(&eventfd_ida, ctx->id);
 	kfree(ctx);
 }
 
@@ -297,6 +303,7 @@ static void eventfd_show_fdinfo(struct seq_file *m, struct file *f)
 	seq_printf(m, "eventfd-count: %16llx\n",
 		   (unsigned long long)ctx->count);
 	spin_unlock_irq(&ctx->wqh.lock);
+	seq_printf(m, "eventfd-id: %d\n", ctx->id);
 }
 #endif
 
@@ -400,6 +407,7 @@ static int do_eventfd(unsigned int count, int flags)
 	init_waitqueue_head(&ctx->wqh);
 	ctx->count = count;
 	ctx->flags = flags;
+	ctx->id = ida_simple_get(&eventfd_ida, 0, 0, GFP_KERNEL);
 
 	fd = anon_inode_getfd("[eventfd]", &eventfd_fops, ctx,
 			      O_RDWR | (flags & EFD_SHARED_FCNTL_FLAGS));